Conventionally, a solid-state imaging element, such as a CCD (Charge Coupled Device), has been extensively used as a camera element of an image pick up apparatus. The resolution of the solid-state imaging element is generally determined by the number of pixels formed on the photo-receiving surface of the solid-state imaging element. Thus, when a high resolution is desired, only the number of the pixels on the solid-state imaging element has to be increased. However, the conventional techniques can increase the number of the pixels only to the extent that the cost and size permit.
To solve the above problem, Japanese Laid-open Patent Application Nos. 54576/1985 and 284980/1988 (Tokukaisho Nos. 60-54576 and 63-284980, respectively), for example, disclose an imaging apparatus adopting an image shifting mechanism. According to these references, an image with a relatively high resolution can be obtained using a solid-state imaging element having a limited number of pixels thereon.
More specifically, as shown in FIG. 16 illustrating the feature of the first reference, a tilting angle .theta. of a plane parallel glass plate 101 is changed, so that incident light on the solid-state imaging element from a subject is shifted by a very small distance .DELTA. when an image is stored into an image memory. For further understanding, a relation between the tilting angle .theta. of the plane parallel glass plate 101 and a shift amount .DELTA. is expressed as: EQU .DELTA.=t.multidot.sin .theta.(1-1/n)
where t is a thickness and n is a refractive index of the plane parallel glass plate 101.
On the other hand, as shown in FIG. 17 illustrating the feature of the second reference, light from a subject 200 goes into a photo-receiving surface of a solid-state imaging element 203 through an optical series 201 and a plane parallel glass plate 202a when an image is stored into an image memory of an image forming section 204. Here, the plane parallel glass plate 202a is fixed to a driving unit 202b, so that the driving unit 202b tilts the plane parallel glass plate 202a at a certain degree angle with respect to the optical axis.
In the prior art, a plurality of images stored in the image memory in the above manner are synthesized, thereby obtaining a resolution as high as the resolution obtained when the number of the pixels on the solid-state imaging element is increased.
Next, the operation for synthesizing shifted images A and B on the image memory will be explained with reference to FIGS. 17 through 20. Since the image synthesizing operations are basically the same, the following description is based on the second reference for convenience.
To begin with, a first image A is stored in the image memory of the image forming section 204, and the alignment of the image data at this point is illustrated in FIG. 18(a). Note that a capital letter A in the drawing indicates each piece of the image data of the image A and numerical subscripts indicate serial column and row numbers assigned to each piece of the image data. For example, A.sub.12 indicates that it is a piece of the image data of the image A aligned at the first column and second row.
Then, the plane parallel glass plate 202a is tilted 45.degree. with respect to the pixel array on the solid-state imaging element 203 both in the horizontal and vertical directions, so that a second image B is obtained by shifting a subject image formed on the solid-state imaging element 203. The image B thus obtained is also stored in the image memory of the image forming section 204, and the alignment of the image data at this point is illustrated in FIG. 18(b). Note that a capital letter B in the drawing indicates each piece of the image data of the image B and numerical subscripts indicate serial column and row numbers assigned to each piece of the image data. For example, B.sub.12 indicates that it is a piece of the image data of the image B aligned at the first column and second row.
Here, the alignment of the image data of the image A with those of the image B is illustrated in FIG. 19. In the drawing, a broken line indicates the image data of the first image data A, while a solid line indicates the image data of the second image data B. The image data of the image B are shifted with respect to the image data of image A by half the pixel pitches Px and Py in X-axis and Y-axis directions of the solid-state imaging element 203, respectively. Here, the X-axis and Y-axis of the solid-state imaging element 203 refer to the horizontal and vertical directions, respectively.
As shown in FIG. 20, the image forming section 204 synthesizes the image data of the two images A and B. In the drawing, positions with a symbol .largecircle. are empty in the beginning, and new image data, for example, an average value of nearby pixels, are provided therein through interpolation later. The resolution of the synthesized image thus obtained is approximately twice as good as the original resolution of the solid-state imaging element 203.
As has been explained, to obtain a high-resolution image by an imaging apparatus furnished with the image shifting mechanism, at least two images must be inputted at different times to effect the image shifting. Note that, however, the images A and B are shifted from each other by a time-related factor besides the above image shifting. The cause of this kind of shifting is assumed to be an unstable vibration conveyed to the imaging apparatus when the imaging apparatus is supported by hands (hereinafter, referred to as hand holding), or the displacement of the subject 200. Since the latter problem is not unique to the imaging apparatus furnished with the image shifting mechanism but common to all types of imaging apparatus, no further discussion is given herein.
Thus, in case that the imaging apparatus, such as a still camera and a movie camera, is not firmly supported on a tripod but supported unstably by hands, the images A and B are shifted from each other by the hand holding in addition to the image shifting, thereby making it impossible to obtain a high-resolution image even when the image shifting is effected. If this kind of unwanted shifting is significant, the resolution of the synthesized image can be hardly improved, and in a worse case, the resolution may be deteriorated.